Much work has been done to remove pollutants from emissions from coal fired furnaces. The focus of most of these efforts has been toward the removal of particulates, nitrogen oxides, or NOx and sulfur oxides, or SOx, from flue gas. Commercially available techniques for reducing nitrogen oxide emissions in furnace flue gases include low-NOx burners, overfire air, selective non-catalytic NOx reduction (SNCR), selective catalytic reduction (SCR), and reburning.
Reburning is a technique whereby a fraction of the total thermal input to the furnace is injected above the primary combustion zone to create a fuel rich zone. Hydrocarbon fuels such as coal, oil, or gas are effective NOx reducers, but non-carbon containing fuels such as hydrogen and ammonia or non-hydrogen containing fuels such as carbon monoxide may also form NHi or HCN intermediates which reduce NOx. Stoichiometry of about 0.90 (10% excess fuel) in the reburn zone is considered optimum for NOx control. Thus, it is apparent that the amount of reburn fuel required for effective NOx control is directly related to the stoichiometry of the primary combustion zone and, in particular, the amount of excess air therein. Under typical furnace conditions of 2% to 4% excess oxygen, a reburn fuel input of over 10% of the total fuel input to the furnace is usually necessary to form a fuel-rich reburn zone. The reburn fuel is injected at high temperatures in order to promote reactions under the overall fuel-rich stoichiometry.
Typical flue gas temperatures at the injection point are 1700K (2600° F.) or greater. Completion air is introduced into the flue gases downstream of the fuel-rich reburn zone in order to complete combustion of any unburned hydrocarbons and carbon monoxide (CO) remaining in the flue gases leaving the fuel-rich reburn zone. In addition, it is also known that rapid and complete dispersion of the reburn fuel in the flue gases is beneficial. Thus, the injection of reburn fuel is frequently accompanied by the injection of a carrier fluid, such as recirculated flue gases, for the purpose of promoting mixing. U.S. Pat. No. 5,746,144 discloses the injection of a coal and water slurry as the reburn fuel. U.S. Pat. No. 6,213,032 discloses injection of an oil and water mixture. U.S. Pat. No. 6,357,367 discloses a biomass and water slurry injection.
To the extent that the recirculated flue gas contains oxygen, the amount of reburn fuel must be increased, because whenever there is proportionally more oxygen than reburn fuel the reburn zone remains overall fuel-lean. However, such fuel-lean reburn provides similar NOx reduction as conventional reburn without the complexity of adding and mixing completion air to complete combustion of the reburn fuel. U.S. Pat. Nos. 5,908,003 and 5,915,310 disclose fuel-lean reburn with gaseous hydrocarbons. The slurry reburn patents U.S. Pat. Nos. 5,746,144, 6,213,032 and 6,357,367 disclose the injection and in-site gasification of various fuel/water slurries, with the subsequent dispersion and mixing of the gasification products used to promote locally fuel-rich reburn zones in an overall fuel-lean flue gas environment.
While the art has focused primarily on the removal of NOx and SOx from flue gas, there are also concerns about emissions of mercury and other elemental metals such as chromium, arsenic and lead from combustion devices. Mercury (Hg), the eightieth element, is an important pollutant. As a vapor in its elemental form, it is a poison of the nervous system. Most industrial uses of mercury today are carefully controlled. The biggest sources of environmental mercury are coal combustion and the combustion of municipal solid waste. Burning coal and especially municipal solid waste compositions may also result in emissions containing chromium, arsenic and lead.
At the levels common in the atmosphere the concentrations are usually safe. However, the mercury accumulates in lakes where it is further accumulated in fish. These fish, with organic mercury molecules in them, can be hazardous to individuals who eat them. Some states request that people eat fish from some lakes no more frequently than once a week. Often it is stated that pregnant women and small children should eat no such fish.
It is known that mercury will combine with chlorine to form mercury chloride and that activated carbon and other fine particulates, such as are present in ordinary fly ash, can capture mercury chloride. But, collection by the use of activated carbon is very expensive. One way to lessen the expense of using activated carbon is to inject raw carbonaceous material into flue gas where the temperature of the flue gas will cause formation of activated carbon. Chang et al. disclose in U.S. Pat. Nos. 6,451,094 and 6,558,454 that one can inject a raw carbonaceous starting material into the flue gas at any location upstream of the particulate collection device in a furnace to form activated carbon particles in the flue gas. The raw carbonaceous material can be injected in dry powdery form or as a wet slurry. These particles may then absorb mercury chloride formed in the flue gas.
Mercury is emitted in power plant flue gases because the elemental form has a relatively high vapor pressure at usual stack flue gas temperature conditions. As such, the elemental mercury is emitted as a vaporous gas, Hg(v), which is very difficult to separate or filter; whereas if the mercury is oxidized it is no longer an elemental vapor. Moreover, the oxidized form exhibits a much lower vapor pressure and tends to collect or adsorb into surfaces of flyash particles or activated carbon particles within the flue gas. Those particles are largely collected before the stack gas escapes.
Mercury does not oxidize to stable concentrations of mercury chloride at temperatures above 1061K (1,450° F.), and oxidation may or may not occur within the temperature range 1005K (1,350° F.) to 1061K (1,450° F.) depending upon gas concentrations and moisture. At temperatures below 755K (900° F.) the rate of oxidation effectively ceases.
Generally, the prior art literature that addresses reduction of NOx and SOx in flue gas says nothing about reduction of mercury or other metals. Conversely, the literature that is concerned with mercury removal, says little or nothing about reducing NOx and SOx. Yet, many coals when burned produce NOx and/or SOx as well as mercury in the flue gas. Consequently, there is a need for a process that will reduce NOx and/or SOx as well as mercury emissions in flue gas.
Preferably, a single treatment step or injection of a single stream of materials should accomplish this reduction. While U.S. Pat. No. 6,726,888 and U.S. Patent Application 2004/0134396 disclose a method to create carbon through inefficient combustion and/or through inefficient completion of reburn combustion, thus at the same time lowering the emissions of NOx, the carbon remaining is the result of the combustion process. This combustion process involves stoichiomtric diffusion flame mechanisms that inherently result in high temperatures and surface vitrification (or ash melting). Thus, even inefficient combustion results in deactivation of the carbon surface and is not an optimum method of producing highly active carbon surfaces. Additionally, these patents involve not a single step, but the interaction of multiple burner, overfire air and/or completion air adjustments.